How do I flip a leg?
First set the pivot point type to 3D Cursor and snap the cursor to the center of the grid (Shift+C). Then in Edit Mode select the whole rig (A). Press Shift+D, RMB. Then press Ctrl+M, type the axis (in my case Y) and -1. It'll mirror your leg along the desired axis. Then select the mirrored rig and press W-->Flip Names. It'll automatically name your legs properly with .R/.L.